drm/i915: add back GEN12_BDSM_MASK

The mask was added in commit e5f415bf ("drm/i915: Add missing mask
when reading GEN12_DSMBASE"), but then looks to be dropped in some
unrelated code movement in commit dbb2ffbf ("drm/i915/mtl: enable
local stolen memory") without explanation. Add it back.

Fixes: dbb2ffbf ("drm/i915/mtl: enable local stolen memory")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221005153148.758822-1-matthew.auld@intel.com